Conventionally, for identification use, various compositions and methods for conducting marking on patterns or characters have been proposed. Patent Document 1 proposes a marking ink composition obtained by adding a pigment to an addition curable silicone compound having a specific composition. Patent Document 2 proposes a marking ink composition obtained by adding a pigment to a condensation curable silicone compound having a specific composition, and a printing method using the same. Both patent documents feature the marking ink compositions having adhesive properties to cured silicone substrates with a rapid cure rate. The base polymers of these marking ink compositions are silicone the same as those of the substrates to be marked, but the compositions are very different from the substrates. Therefore, the surface properties of the substrate covered with such a marking composition are different from those of the substrate. According to the intended use, the properties of the substrate are adjusted to be optimum by selecting materials having suitable properties or by customizing the composition. For example, such properties include suitable flexibility to follow vibrations and temperature changes, and compressibility for maintaining sealing properties and heat conductivity. The suitable properties according to the intended use often include the surface properties as well as the bulk physical properties. For example, the surface properties such as an adhesive properties and tackiness are crucial in maintaining the sealing properties and the heat conductivity. Nevertheless, marking is for the purpose of identification and is irrelevant to the suitable properties of the substrate according to the intended use. In fact, however, marking can deteriorate the properties of the substrate, particularly the surface properties of the substrate. For example, the adhesive properties and tackiness of the substrate surface can be decreased by marking. This is a problem in conducting marking on silicone-based materials or components, and had not been solved.
Various phosphor-containing curable silicone compositions have been proposed in recent years. These numerous proposals, Patent Document 3 as an example, relate to a phosphor-containing silicone composition for use in converting LED light colors. Similar to Patent Document 1 and Patent Document 2, the composition of Patent Document 3 is curable, and therefore does not provide a solution to the problem of conducting marking without changing the surface properties of the substrate. On the other hand, the usability of a so-called invisible ink composition has been recognized. Therefore, the invisible ink composition can easily be identified by ultraviolet light irradiation while it is almost invisible to the naked eye in a visible light region. In the visible ink composition, a phosphor that emits visible light when irradiated with ultraviolet light is used instead of a pigment. In particular, the great advantage of the invisible ink composition is that identification can be conducted easily by ultraviolet light or black light irradiation without the use of the invisible ink being noticed by others. For silicone-made industrial assembly parts related manufacturing and supplying industries, the detection of counterfeits is a typical example of the advantage. Patent Document 4 is an example of the use of such invisible ink. Although the example of Patent Document 4 shows only a vinyl chloride polymer as a resin component of the invisible ink and a substrate, a silicone polymer is also exemplified to be effective. However, the resin component of the invisible ink has been assumed to be curable. Therefore, the problem of conducting marking without changing the surface properties of the substrate remains unresolved when the invisible ink is applied to a silicone-based material. Patent Document 5 proposes water-based invisible ink for inkjet printing. However, it is difficult to apply the water-based invisible ink to the silicone material.
A technique not for identification purposes has been proposed in which a rubber layer is formed by increasing a cross-linking density in the vicinity of the surface of a silicone gel material (Patent Documents 6 and 7). In this technique; first, a silicone having a SiH group as a cross-linking component for an addition curing is applied to a release film before curing an addition curable silicone gel, which is to be a substrate. Then, the release film is disposed on the substrate, and heated and cured to produce a molded product. However, this is an opposite approach to solve the problem of conducting marking without changing the surface properties of the substrate.
Patent Document 1: JP-S52 (1977)-14642 B
Patent Document 2: JP-S57 (1982)-49589 B
Patent Document 3: JP-4949130
Patent Document 4: JP 2002-169469 A
Patent Document 5: JP 2012-111946 A
Patent Document 6: JP-H10 (1998)-6440 A
Patent Document 7: JP-H10 (1998)-183110 A